Looks like an artillery hit. Something right from the turner diaries.
Smart because unstoppable and hard to track. Even in our modern surveillance prison mortar / artillery would be hard to track.
The only two ways I can think of tracking something like that are radar detection systems that are active when the shot is fired (in order to obtain a trajectory), or a ShotSpotter-like system to detect the shot and triangulate a position.
Safe to say neither of those would be present in the fields of Georgia.
Could radar really detect the trajectory of basically a slightly large bullet?
Thing is if you could fit it into a truck you’ve got potentially miles or more from around the target to choose from.
I guess microphones could triangulate the origin of the boom but you get to choose the less likely area to have them.
Also maybe you could design a boomless “flinger”. Like this maybe -
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